DESCRIPTION: The sequential action of uroporphyrinogen (uro'gen) decarboxylase and coproporphyrinogen (copro'gen) oxidase transforms uro'gen-III via copro'gen-III into protoporphyrinogen-IX (proto'gen-IX). This section of the heme biosynthetic pathway has been little studied in recent years, even though a number of ambiguities remain. Very recently, the action of uro'gen decarboxylase on uro'gen-III has received some renewed attention due, in part, to conflicting results on the pathway for the sequential decarboxylation process. The stepwise conversion of copro'gen-III to proto'gen-IX has been unambiguously shown to involve a single pathway, although the mechanism for the oxidative decarboxylations of the propionate sidechains is presently unknown. Using synthetic porphyrinogens as probes, the substrate specificity of crude and purified copro'gen oxidase will be examined in detail and a model for the active site of this enzyme will be rigorously tested. Analogs of coproporphyrin-III and harderoporphyrin will be synthesized, including harderoporphyrin-I, and the metabolism of the corresponding porphyrinogens by copro'gen oxidase will be investigated. The action of copro'gen oxidase on the four pentacarboxylate porphyrinogens related to uro'gen-III will be studied, and this work may lead to the identification of novel abnormal pathways for heme biosynthesis. These studies will have clinical implications, as abnormal porphyrin metabolites are excreted in the porphyrias (diseases of porphyrin metabolism). The steric requirements for the active site will also be examined with O-ethyl and meso-methyl analogs of copro'gen-III. In related studies, the mechanism of the oxidative decarboxylation of copro'gen-III will be studied. Trace amounts of a hydroxypropionate porphyrinogen are known to accumulate in incubations of copro'gen-III with copro'gen oxidase. The investigator will isolate the corresponding porphyrin and examine the stereochemistry of this system. Porphyrinogens with isobutyrate or cyclopropyl sidechains will be synthesized and these will provide novel mechanistic probes for copro'gen oxidase. Molecular dynamics will be utilized to investigate the conformations of porphyrinogens. Finally, cycloalkanoporphyrinogens will be used to further investigate the conformational requirements for this poorly understood enzyme.